The nature of the seventy sevens
One of the great questions about the 70 weeks prophecy of Daniel 9, that Bible scholars have sought to answer, is why did Daniel specify the time to the appearance of the Messiah as divided up into two sections? Daniel represents this time by seven weeks, and sixty and two weeks. Why didn’t he simply say sixty-nine? In most expositions, the question is left unanswered, and is usually ignored.
In the discussion by O. Palmer Robertson quoted below, the problem is stated: “some reason must be given for the breakdown of Daniel’s seventy sevens into three periods consisting of seven sevens, sixty-two sevens, and one seven.” Robertson suggested that symbolic time periods included in the 70 weeks may resolve it. 
The nature of the seventy sevens
It might be presumed that the seventy sevens of Daniel 9 should be regarded purely from a figurative perspective, in light of the broader scriptural role of the Sabbath concept in redemptive history. Indeed, the symbolic character of the number seven as an aspect of the Sabbath concept must not be ignored. The perfection of sevens as embodied in the “seventy sevens” speak of the movement toward the final climax of the Covenant Lord’s redemptive work in the world, The Sabbath rest that remains for the people of God must be seen as the ultimate goal of the seventy sevens (Heb. 4:9).
At the same time, the context in which Daniel’s prophecy is found inevitably points to an actual chronological ordering in the purposes of God, Jeremiah did not predict Israel’s banishment from the land to be, for example, twenty-three years or forty-one years in length–he predicted seventy years. While the overscrupulous may bicker about the precise beginning and ending of the seventy years, the span still stands quite definitively. Seventy years prior to 536 BC, takes us back to approximately 605 B.C., which served as Jeremiah’s marker for the beginning of the seventy years of exile (Jer. 25:1-3, 11).
If a specific chronological ordering is manifest in the seventy years of Israel’s captivity as predicted by Jeremiah, and if this principle of seventy in the earlier portion of Daniel 9 has a significant connection with the seventy of the latter portion of the same chapter, it seems likely that the “seventy sevens” also have some chronological significance. In addition, some reason must be given for the breakdown of Daniel’s seventy sevens into three periods consisting of seven sevens, sixty-two sevens, and one seven. A purely figurative analysis is totally at a loss to explain this subdivision of the sevens; and yet the breakdown clearly has significance in Daniel’s vision.
Resolution of the question concerning the nature of the seventy sevens may be found in an inclusion of the symbolic in the chronological. The larger picture of movement through history toward the climax of God’s redemptive purposes in the rest, the Sabbath that remains for the people of God, must not be forgotten. At the same time, some effort must be made to determine the intentional chronological ordering of the ages.
Robert B. Chisholm Jr. noted that the seven weeks and sixty two weeks are separate and distinct in the Hebrew text of Daniel 9:25. He wrote: 
The text reads literally “seven weeks, and sixty-two weeks.” Some combine the numbers and understand the text to mean “until an anointed one, a ruler [arrives], [there will be a period of] sixty-nine weeks.” However, this would be an odd way of expressing the number sixty-nine. Elsewhere, numbers in the sixty range are expressed by combining “sixty” with the other number. For example, sixty-two is literally “two and sixty” (Dan. 5:31) or “sixty and two” (Dan. 9:25-26), sixty-five is “sixty and five” (Isa. 7:9), sixty-six is “sixty and six” (Gen. 46:26; Lev. 12:5), and sixty-eight is “sixty and eight” (1 Chron. 16:38). The traditional scribal punctuation of Dan. 9:27 marks a clear break between “seven weeks” and “sixty-two weeks.” The latter is best taken as a temporal adverbial phrase that begins the following clause. The reference to “sixty-two weeks” (not “seven weeks and sixty-two weeks”) in v. 26 shows the sixty-two weeks are understood as distinct from the seven weeks.
In an article surveying the interpretations of Daniel’s prophecy, Elke B. Speliopoulos quoted Chisholm as saying: 
There seems to be no point in saying ‘seven weeks and sixty-two weeks’ unless something is going to happen after the seven. Moreover, the repetition of ‘sixty-two’ in v. 26, ‘After the sixty-two weeks, an anointed one shall be cut off’, also suggests that the two periods of seven and sixty-two are discrete ones, separated by something.
The requirement of different units in the first two sections of the seventy weeks provides the simplest explanation for distinguishing between the seven weeks and the sixty-two weeks. The first section of the 70 weeks consists of seven weeks of leap years, that is, years of 13 months. Each week in the first section spans 19 years, and 49 leap years spans 133 years. In the second section, the units are either sabbatical cycles of seven years, or seven weeks of sixty-two years, (the age of Darius when he became ruler of Babylon) and spans 434 years. Together these two periods end at the appearance of Christ in 28 AD, when the decree of Cyrus in 538 BC is used as the beginning of the 70 weeks.
Another reason the period to the coming of Christ is specified in two sections is that the first two sections of the 70 weeks prophecy correspond to the second and third of the four periods of seven times in Leviticus 23. The correspondence is illustrated in the table below.
A chart of the 7 times of Leviticus 26 and the 70 weeks
|First 7 times
|70 years of exile in Babylon
2 Chronicles 36:17-23; Jeremiah 25:11-12; Jeremiah 29:10
|Second 7 times
|7 weeks, Daniel 9:24-27
Units are leap years; years of 13 months
7 x 7 leap years = 133 years
|Third 7 times
|62 weeks, Daniel 9:24-27
62 x 7 years = 434 years
405 BC – 28 AD
|Fourth 7 times
|1 week, Daniel 9:24-27
Christ confirms the covenant for 1 week, having two parts: Jesus’ ministry on earth, plus a symbolic half-week, represented by ‘a time, times, and a half,’ Revelation 12:14; 1,335 days, Daniel 12:12; 1,290 days, Daniel 12:11; 42 months, Revelation 11:2, 13:5; 1,260 days, Revelation 11:3, 12:6. The last half-week spans the whole time from 30 AD to the end of the age
It is in the fourth period of seven times that the people become reconciled to God, and God remembers his covenant with Abraham, Isaac, and Jacob, and “remembers the land.” [Leviticus 26:42]
Robertson pointed out that the final week of the 70 weeks prophecy is also divided into two sections. And units form the basis for the division in this case also; the units in the last half-week are symbolic. He wrote: 
This final week, unlike all the previous weeks, is divided into two halves: “In the midst of the seven [i.e., at the halfway point of the seven; khatsi], he will cause sacrifice and offering to cease” (9:27). This cessation of sacrifice corresponds with the atoning of iniquity and the bringing in of eternal righteousness (9:24) accomplished earlier in the seventy sevens. Once iniquity has been covered by a proper atonement, there can be “no more sacrifice for sin” (Heb. 10:26). This climactic event, according to Daniel 9:27, is to occur at the halfway point of the seventieth week of seven years, or 3.5 years into the last week.
The figure of 3.5 years receives further development in the final chapter of the book of Daniel and even more extensively in the book of Revelation. In his final interview with the revealing person, Daniel overhears the question, “How long will it be before these astonishing things are fulfilled?” (Dan. 12:6 NIV). The man clothed in linen takes a solemn oath that the period will be “for a time, times, and half a time,” reflecting the same earlier measurement of the time that the saints will suffer at the hands of the little horn of the fourth beast of Daniel 7 (12:7; see also 7:25). This same measurement recurs in the form of 1,290 (or 1,335) days that are to expire between the time that the daily sacrifice is abolished and the abomination that causes desolation is set up (12:11-12).
Contrary to what Robertson stated above, the 1,290 days are not the time unto the setting up of the abomination of desolation, but the time remaining after that event, which therefore occurs early in the church age. Both the 1,290 days, and the 1,335 days run to the end of the age, and are symbolic. The 1,290 days, being the smaller number, is a portion of the greater period. Similarly, the 1,260 days in Revelation are a yet smaller portion of the same period. The apostle John declared that there were already many antichrists already in his time. [1 John 2:18]
Robertson wrote: 
The various ways in which this last half of the final week is designated in Daniel suggest that the time measurement has been modified from the chronological/symbolical to the purely symbolical. The book of Revelation reflects the same diversity in referring to an identical period as a symbolical device: the 1,260 days, 42 months, and “time, times, and half a time” (Rev. 11:2-3; 12:6, 14; 13:5). The last half of the seventieth week of Daniel may thus be regarded as a different form of time measurement. This last half-week symbolically represents a longer period of time, indefinite in length, which extends from the ending of sacrifice until the destruction of the antichrist at the consummation of the present age. During this last epoch of time, if the book of Revelation is allowed to provide some insight, the true people of God will be persecuted relentlessly as they bear witness throughout the world. But they will also be protected by the providential orderings of the Lord (11:2-3; 12:6, 14; 13:5). At the end, all enemies of the Lord will be destroyed at his coming.
Here is a reason for the change in the nature of time units in the final half-week: that portion of the prophecy applies to the heavenly city, not the earthly one. The previous part of the prophecy employs earthly units, because they apply to the earthly Jerusalem. They consist of natural years, and leap years, units which would not apply to the heavenly Jerusalem. Rather, in the heavenly city, the time is represented symbolically. The months are not real lunar months, but ideal months of exactly 30 days. Daniel represents it by the numbers 1,335 days, and 1,290 days, which do not fit a real three and a half years in any calendar. And a real three and a half years cannot equal two different numbers of days. These numbers represent symbolic periods consisting of leap years of 13 months and regular years of 12 months, the first year differing from the rest in each case. The 1,260 days of Revelation 11 and 12 is based on the same pattern, but all years are 12 months.
3. Elke B. Speliopoulos, The 70 weeks of Daniel: a survey of the interpretive views